1. Field of the Invention
This invention relates generally to cooling systems for internal combustion engines and more particularly is directed to a liquid cooling system for and to the conversion of such engines from air cooling to liquid cooling.
2. State of the Prior Art
Many light aircraft in current service are powered by horizontally opposed piston engines. This type of engine is characterized by multiple pairs of piston cyclinders, each pair being mounted to opposite sides of a common crankcase block with all of the cylinders lying in a common horizontal plane. This type of engine is most notably exemplified by the Lycoming series of aircraft engines, and also certain engines made by Continental. The Lycoming engines are made in four cylinder configurations and to a lesser extent in six and even eight cylinder configurations, and are in widespread use in the civil aviation and light aircraft community. These engines have gained wide acceptance and have remained essentially unchanged since about 1955. For purposes of this disclosure reference is made primarily to Lycoming engines because these are the most prominent example of the type of engine to which this invention is directed. It should be understood, however, that the liquid cooling system and conversion according to this invention is not limited to any particular make or brand of engine, nor for that matter, to aircraft engines. Aircraft engines have discrete cylinders each individually bolted to a common crankcase block. This is distinguished from an in-block cylinder engine where the cylinders are contained in a common engine block.
The Lycoming engine in its original factory configuration is cooled by an air stream produced by the turning propeller driven by the engine. Air intakes defined by a cowling arrangement around the engine admit propeller driven air from the atmosphere into the engine compartment and over the piston cylinders on either side of the engine. The air heated through contact with the engine is discharged to the atmosphere through vent openings in the fuselage. Each piston cylinder includes a cylinder sleeve which contains a reciprocating piston and a cylinder head which is assembled to the outer end of the cylinder. The cylinder head closes the top or outside end of the cylinder and also carries the intake and exhaust ports and valves which control the flow of the air/fuel mixture into the cylinder and the hot exhaust gases out of the cylinder. The cylinder head also carries the spark plug or plugs which ignite the air/fuel mixture. A system of push rods external to the cylinders and driven by a crank turning in the engine block actuates the intake and exhaust valves on each cylinder through a rocker assembly in the cylinder head in time with an electrical ignition system which fires the spark plugs. The exterior surfaces of the cylinder and the cylinder head carry a series of annular radiator fins which greatly increase the metal surface exposed to the air stream and thereby enhance the transfer of heat from the cylinder to the air stream.
The Lycoming engine also has an accessory pad on the crankcase block with an output drive shaft which conventionally provides a power take-off for various accessories such as an engine governor or a propeller pitch drive.
Air cooling of aircraft engines has proved popular because it eliminates the weight and reliability issues of the radiator, pump and hoses of a liquid cooling system. On the other hand, air cooling suffers from a number of disadvantages as well. Firstly, air flow through the engine compartment and against the cylinders introduces significant drag, Secondly, cooling of the various cylinders is uneven, some receiving significantly better airflow than others depending of the position of each cylinder and the cowling configuration in the particular fuselage. Thirdly, air cooled Lycoming and similar aircraft engines operate at elevated temperatures, typically in the range of 400-500xc2x0 F. and, although the engines are rated at 2000 hours before overhaul is needed, in actuality these engines have substantially shorter service lives. The conventional air cooled cylinder heads have a very large temperature differential across the head, between the intake valve and exhaust valve sides of the head. The intake side is cooled by the relatively cold air/fuel mixture flowing into the cylinder, while the hot combustion exhaust gases typically have a temperature of about 1500xc2x0 F. The result is a differential of some 200xc2x0 F. across the cylinder head, which often leads to cracking of the head within some 1100 hours of engine operation. This temperature differential can be reduced to about 25xc2x0 F. by water cooling the cylinder head. Shock cooling of the cylinders may occur in a nose down descent with the engine running at idle, where rapid air flow can cause a rapid drop of 200xc2x0 F. in cylinder head temperature while little heat is generated during idle operation, causing warpage of both the cylinders and the cylinder heads as one side shrinks relative to the other, the cylinders go out of round. Conversely, shock heating of 200xc2x0 F. to as much as 400xc2x0 F. of the cylinder head can happen during engine run-up prior to takeoff while the aircraft is stationary but developing high r.p.m. with little air flow over the engine. At temperatures of about 320xc2x0 F. and above the aluminum alloy of the cylinder head looses T6 hardness and becomes more susceptible to cracking. Critical failures involving cracks developing in the cylinder heads and sticking of exhaust valve stems become more likely under such circumstances. Air cooling cannot sufficiently cool the exhaust valve area leading to carbonization of valve stem lubrication oil. These carbon deposits eventually lead to valve sticking. Also, repeatedly raising and lowering the aluminum alloy temperature induces work hardening of the metal and is also a factor leading to cracking of the cylinder heads.
Liquid or water cooling, on the other hand, is conducive to lower engine operating temperatures and more even cooling of all engine cylinders with lower air cooling drag. An estimated ten percent increase in air speed is obtainable by converting a given air cooled engine to liquid cooling, while at the same time reducing engine operating temperature to approximately 190xc2x0 F. In turn, reduced engine temperatures permit an increase in engine compression ratio which translates into higher engine power output. Also, lower engine temperatures allows the engine to be run at lean fuel mix at low altitudes, even at sea level, without detonation and at higher power output than is possible with air cooling of the engine. A rich fuel mix, e.g. 19 gallons of fuel per hour (full rich), also operates to cool the engine, whereas a lean fuel mix such as 10 gallons per hour (a typical cruise lean mix) is more susceptible to detonation due to high engine temperature at oxygen rich low altitudes. Liquid cooling of the engine greatly reduces the chances of such detonation because of markedly lower combustion chamber surface temoertures.
A large number of light aircraft are in service with air cooled horizontally opposed piston engines which could benefit from conversion to liquid cooling. There is also a need for robust yet easy to install power plants in the experimental aviation, which presently relies on small, low power air cooled engines or, for higher power applications, on converted automobile engines which tend to be too heavy and run too fast for aircraft use. Heretofore, however, no conversion from air cooling to liquid cooling has received certification by the FAA because of the cost and difficulty of the certification process.
Many attempts have been made in the past to convert air cooled piston engines of various types to liquid cooling. However, because of the all important need for dependability in aircraft engines these attempted conversions have not found acceptance in the aviation industry, and only engines designed from the ground up for liquid cooling have found use in the aviation field.
Exemplary of past efforts at conversion to liquid cooling are the patents issued to George U.S. Pat. No. 4,108,118; Wintercorn U.S. Pat. No. 1,725,121; and Ronen U.S. Pat. No. 5,755,190. George provides a water cooled replacement for an air cooled cylinder, but retains the air cooled cylinder head. Further, the replacement cylinder is encompassed by a water jacket made up of two semi-cylindrical halves which require difficult and unreliable sealing to each other and to the cylinder sleeve. Wintercorn provides water cooling by fitting a cylindrical container over the air cooled cylinder sleeve and circulating liquid coolant through the enclosed space defined between the sleeve and the outer container. The outer container does not cover the cylinder head which remains air cooled. Also, this approach suffers from the same sealing problems as the George conversion and is unsuitable for aircraft use. Ronen describes a more comprehensive solution by replacing the cylinder head with a replacement head which features internal coolant passages and an integral jacket which extends over the cylinder sleeve. Nonetheless, the Ronen conversion still requires problematic sealing of the jacket to the cylinder sleeve. Yet another source of difficulty with each of the three prior patents is the possibility of electrolytic corrosion between the external water jacket and the cylinder sleeve if these two elements are of different metallic composition. These prior art patents also fall short in that problems specific to conversion of multi-piston engines and to providing adequate cooling to the very hot exhaust side of the cylinder heads are not addressed. Water manifolding and coolant circulation within the cylinder is key to successful water cooling of the cylinders in the aircraft engine. These and other shortcomings render prior attempts at conversion to liquid cooling unsuitable for implementation in aircraft power plants.
A continuing need exists for a reliable liquid cooling system for horizontally opposed piston engines useful for conversion of existing air cooled engines and also for implementation as original equipment in newly manufactured engines.
The present invention addresses the aforementioned need by providing a method and components for a liquid cooling system for horizontally opposed piston engines and particularly but not exclusively for Lycoming horizontally opposed piston aircraft engines.
In its broader aspect this invention provides a minimally invasive method of converting to liquid cooling a horizontally opposed piston engine having air cooled finned piston cylinders mounted to a common crankcase block and air cooled cylinder heads on the finned piston cylinders. The novel method involves the steps of detaching each of the finned piston cylinders from the crankcase block together with the air cooled cylinder heads and substituting therefor a liquid cooled replacement cylinder comprising a unitary casting including a double walled jacket defining an annular coolant cavity having an open end and an opposite end closed by a head portion having intake and exhaust ports, the head portion including coolant passages in fluidic communication with the annular coolant cavity, and a coolant inlet and a coolant outlet on the jacket for circulating coolant through the coolant cavity and the coolant passages, and a cylinder sleeve fitted to the open end of the double walled jacket; mounting a coolant pump on an accessory pad of the engine and connecting an accessory drive shaft of the accessory pad for driving the pump; providing a radiator; and interconnecting the pump, the radiator, and the coolant inlet and coolant outlet of each replacement cylinder to make a closed coolant circuit.
The method of this invention may also include the step of orienting each replacement cylinder relative to the crankcase block such that each coolant inlet is near a lowermost point along a circumference of the annular coolant cavity and each coolant outlet is near an uppermost point along a circumference of the annular coolant cavity on each of the horizontally opposed pistons, whereby coolant flow through the annular cavity of each replacement piston is in a generally upward direction from the coolant inlet to the coolant outlet and convective flow of coolant through the annular cavity is maintained in the event of failure of the pump to thereby delay overheating of the engine.
This invention also contemplates a liquid cooled internal combustion engine having plural pairs of horizontally opposed pistons, each piston displaceable in a piston cylinder external to a common crankcase block, the engine assembled with each piston cylinder having a unitary casting including a double walled jacket defining an annular coolant cavity having an open end and an opposite end closed by a head portion having intake and exhaust ports, the head portion including coolant passages in fluidic communication with the annular coolant cavity and arranged for directing coolant into thermal proximity with the exhaust ports and returning coolant to the annular coolant cavity, and a coolant inlet and a coolant outlet on the jacket for circulating coolant through the coolant cavity and the coolant passages; a cylinder sleeve fitted to the open end of the double walled jacket; a radiator; and a pump directly gear driven by an accessory drive shaft of the engine for circulating coolant liquid through the unitary casting of each piston cylinder and the radiator thereby to dissipate heat from the piston cylinders to the environment through the radiator. The liquid cooled engine has an accessory pad and an accessory drive shaft on the crankcase block, the pump being mounted to the accessory pad and driven by the accessory drive shaft. The pump further comprises a step-up gear assembly between a rotor of the pump and the accessory drive shaft whereby the pump rotor turns at higher speed than the accessory drive shaft.
A more particular aspect of this invention is a replacement cylinder for use in providing liquid cooling to an air cooled internal combustion engine of the type having one or more piston cylinders exterior to a crankcase block. The replacement cylinder features a unitary casting including a double walled jacket defining an annular coolant cavity having an open end and an opposite end closed by a head portion having intake and exhaust ports, the head portion including coolant passages in fluidic communication with the annular coolant cavity, and a coolant inlet and a coolant outlet on the jacket for circulating coolant through the coolant cavity and the coolant passages; and a cylinder sleeve fitted to the open end of the double walled jacket. Preferably the cylinder sleeve is threaded to the unitary casting, the cylinder sleeve and unitary casting are of materials having dissimilar coefficients of thermal expansion, and the cylinder sleeve and unitary casting are fitted to each other in a compressive interference fit by differential thermal expansion. In the preferred for of the invention the cylinder sleeve and the unitary casting are threaded to each other and permanently joined in a fluid tight interference fit resulting from differential thermal contraction during cooling following hot assembly of the two parts. The unitary casting is preferably of aluminum and the cylinder sleeve is of steel.
The double walled jacket of the unitary casting has an outer wall and an inner wall both joined to the head portion and further joined along a common bottom, the annular coolant cavity being defined between the outer wall and the inner wall, with the inner wall being in thermal contact with a substantial portion of the cylinder sleeve such that coolant liquid circulating through the cavity cools the cylinder sleeve without coming into contact with the cylinder sleeve, whereby electrolytic corrosion is avoided between the casting and sleeve of dissimilar metals.
The double walled jacket may have interior fluid gating configured for diverting a substantial portion of coolant liquid entering the inlet into the coolant passages of the head portion thereby to provide liquid cooling to the exhaust port portion of the cylinder head. The fluid gating may preferentially divert coolant entering the jacket inlet into the head coolant passages over the annular coolant cavity.
It is preferred that the coolant inlet be near a lowermost point along a circumference of the annular coolant cavity and that the coolant outlet be near an uppermost point along a circumference of the annular coolant cavity on each of the horizontally opposed pistons, whereby coolant flow through the annular cavity is in a generally upward direction from the coolant inlet to the coolant outlet and convective flow of coolant through the annular cavity is maintained in the event of failure of the coolant pump thereby to delay overheating of the engine.
Yet another aspect of this invention is a cooling system instrumentation system and display having:
a) a temperature indicator driven by a temperature sensor in thermal contact with the coolant liquid;
b) an actual water pump outlet pressure indicator driven by an input signal representative of the difference between an instantaneous pump outlet pressure and a coolant static or system pressure measured at a point downstream from the pump and upstream of the engine; and
c) a low coolant indicator actuated by a signal representative of a relatively low coolant system pressure coupled with a relatively high coolant temperature.
The point downstream may be at a thermostat connected downstream of the pump for controlling coolant flow through or for bypassing the radiator, and the relatively low coolant system pressure is desirably an adjustable pressure. For example, the relatively low coolant system pressure may be a pressure lower than 5 psi and the relatively high coolant temperature may be greater than 160xc2x0 F.
Still another aspect of the liquid cooled engine according to this invention is a coolant manifold comprising a T-fitting including a center tube attached to each coolant inlet and coolant outlet of the double walled jacket of the unitary casting, and a cross tube open at opposite ends; a ring seal at each of the opposite ends, a connecting tube inserted into the ring seals of mutually facing open ends of adjacent ones of the piston cylinders, and a hose connected to a first one of the cross tube ends and a plug closing a last one of the cross tube ends, one hose being connected to an outlet of the pump for delivering coolant to the cylinders, the other hose being connected for returning hot coolant to a thermostat. Preferably the T-fittings and the connector tubes are made of aluminum for lightweight.